The present invention relates to apparatus for separating fine and coarse particles from each other and in particular to a counter-flow pneumatic separator of loose granulate or particulate matter.
In general, pneumatic separators comprise a vertical feed pipe for bulk granulate or particulate material which is fed into the top end of a separating chamber defined by a housing provided at its upper end with a discharge conduit and at its lower end with an air feed conduit, providing air under selected pressure in an upward direction.
Such a counterflow separator is known from the German DE-OS 19 05 106. With it, bulk material is classified, i.e. separated, without prior handling directly from the feed pipe. Since the sifting or separating air flows countercurrent to the bulk graunulate or particulate mixture, great relative speeds result between the certain components of the mixture and the classifying air, leading to correspondingly large tear-off or stress forces which will safely separate dust particles adhering to granulates, pellets etc. This separation occurs in the area between the opening mouth or orifice of the feed pipe in the chamber and the housing's opening at its lower end which leads into a collecting vessel arranged below it. This area is termed the braking stretch and the fine material is braked there by the classifying counter flowing air current and, with reversal of its direction of movement, carried upward by the combining currents of the delivery and classifying air by way of the annular clearance between the part of feed pipe projecting into the chamber and the housing wall and discharged through the discharge conduit.
The particle size, up to which the fine material is separated, is determined by two factors, namely on the one hand by the velocity of the classifying air, and on the other hand, by the length of the braking stretch. Although, as detailed studies have shown, a long braking stretch is desirable in principle because it reduces the necessary amount, and speed of the classifying air and therefore the energy required for the separation, the length of the usually cylindrical braking stretch in the known countercurrent classifier is limited. This limitation is created by the requirement that not only does the course of the particles of fine material have to be changed in such countercurrent separation, but that the particles of coarse material too, entering at first in the axially moving current, experience a certain degree of lateral deflection leading to the movement of the stream of coarse material from the vertical path given it as it exits from the material feed pipe. This deflection has the effect of causing the impact of particles of coarse material on the housing wall, the number of such impacting particles increasing with the increase in length of the braking stretch. As a result, the impacted particles lose so much kinetic energy that they are caused to be carried out through the fine discharge conduit with the countercurrent air composed of the incoming classifying and delivery air, thus causing the fine material to contain an inadmissibly high share of coarse material.
The invention is based on the task of reducing the amount of classifying air and energy required with a countercurrent separator of the type described at the outset, while maintaining the classifying quality the same.
These objects as well as others will be apparent from the following disclosure of the present invention.